We describe a system and method capable of projecting images and, more particularly, a system and method of keystone correction.
Use of digital projection systems for lecture, presentation environments, and for home entertainment continues to increase. The projection system, including a projector, casts a digital image onto a planar screen, wall, or other display. The projector may project a still frame, such as a slide or photo. Or the projector may project a video signal, comprising a rapid succession of frames. Projector electronics process the digital image, and then output a corresponding pixelated image from the projector's optical system. A projection lens controls projected image size and focus. A projection axis is defined as an axis along the center of projection lens to the center of the projected image. The projection lens is usually offset from an optical axis to raise a projection axis well-above horizontal for viewing convenience.
If the projector system casts the image onto a screen or display that is not perpendicular to the projection axis, the projected image will distort. That is, the image will appear stretched, deformed, or otherwise misshapen. This distortion is called keystone because the distorted projected image may take on the architectural shape of the same name.
FIG. 1 shows a projector 10 tilted above horizontal, causing its projected image 20 onto a screen 15 to be keystone distorted into a trapezoid (indicated by the dashed line).
FIG. 1 also shows a keystone-corrected image 25, corrected by a horizontal scaling. Keystone distortion caused by an up or down tilt of the projector is correctable by a horizontal scaling of the digital image. Keystone distortion caused by a right or left panning of the projector is correctable by a vertical scaling of the digital image.
Horizontal or vertical scaling a digital image tends to be memory intensive. For example, a single XGA (resolution) frame has 1024×768 pixels, each pixel having encoding for the three primary colors. Any process that manipulates a video signal, which may consist of 20 to 30 frames per second, needs to work fast and efficiently frame-by-frame. To this end, a frame buffer may be used in conjunction for scaling to store an input image or an intermediate image. A frame buffer, however, is costly to implement.
Accordingly, a need remains for an improved keystone correction system and method.